1. Field of the Invention
The invention of this application relates to a method and apparatus for hemodialysis, and more particularly to a method and apparatus for treating patients having a large body mass.
2. Description of the Prior Art
In the field of renal dialysis, the use of dialyzer cartridges to remove blood borne-uremic toxins and by-products of metabolism has been conventional for many years. Typically, such a cartridge contains essentially two chambers separated by a semipermeable membrane. Blood is perfused through the first chamber and returned to the patient. A dialysis solution is simultaneously circulated through the second chamber. A concentration gradient is thereby established which causes toxic molecular species contained in the blood to migrate through the semipermeable membrane and into the dialysis solution.
The principle of hemodialysis has been refined extensively. Dialyzers which contain a large plurality of semipermeable hollow fiber membranes are now utilized to greatly increase the surface area, thus facilitating diffusion across the membrane. The hollow fiber membranes are composed of a variety of materials including cellulose acetate, cellulose triacetate, polyacrylonitrile, polysulfone and regenerated cellulose. These hollow fibers are small bore capillaries arranged in parallel. The bundle of fibers is typically potted in a curable sealant at both ends. Upon cure, the embedded fibers are cut through at the ends to expose the bores of the fibers, as disclosed in U.S. Pat. No. 4,227,295 (Bodnar et al.), incorporated by reference herein. The fiber bundle is then enclosed in a housing which forms the dialyzate chamber. Examples of dialyzers of this construction together with the mechanical details of closure, inlet and outlet ports, and the like are illustrated in U.S. Pat. No. 4,283,284 (Schnell) and U.S. Pat. No. 4,600,512 (Aid), both incorporated by reference herein.
In the operation of the dialyzer, patient blood is pumped through the hollow fiber bundle, and a dialysis solution is pumped through the dialyzate chamber so that dialysis solution constantly bathes the exterior hollow fiber surfaces. Pump assisted movement of blood through the dialyzer is required in order to displace a sufficient volume for effective cleansing within a treatment time of less than six hours. Another pump propels dialysis solution through the dialyzate chamber and also regulates, by valves under microprocessor control, the proper mixing of dialysis solution concentrate and water. Electrolyte concentrations are monitored by continuous conductivity measurement. Another function of this pumping device is to carefully control fluid back pressure so as to prevent excessive water loss from the blood. Examples of such units are disclosed in U.S. Pat. Nos. 4,137,168, 5,344,568, and 5,744,027, all of which are incorporated by reference herein.
One measure of adequacy of dialysis for the individual patient as to a particular dialyzer is calculated using Daugirdas' equation: EQU Kt/V.gtoreq.0.8
wherein V is an expression of the volume of distribution of urea which is approximately equal to total body fluid volume derived for each individual patient from data such as height, weight, and sex, K is the urea clearance of the dialyzer in use, in ml of blood totally cleared in urea each minute, and t is the treatment time. A typical product insert accompanying a dialyzer unit contains a graph of urea clearance versus blood flow rate obtained by random testing of a sample of dialyzers from a particular manufacturing lot. Upon incorporating these values into the above equation, the minimum treatment time can be calculated for a given Kt/V value. Other parameters that may be varied to achieve adequate dialysis include blood flow rate (BFR), dialysis solution flow rate (DFR), dialyzer competency, and temperature.
It has been determined empirically that Kt/V values of about 0.8 or greater are associated with low levels of morbidity. See Gotch, L. A., Sargent, J. A., Kidney Int., 28:526-537, 1985. Even in the use of new dialyzer units there is some risk that a unit selected from a particular lot will have a significantly lower K value than the value depicted by the product insert graph. The patient receiving treatment from such a unit is therefore at risk of being under dialyzed. The likelihood of under-dialysis increases upon reuse of the dialyzer because of the unpredictability of loss of dialyzer competence.
Dialysis adequacy in patients weighing greater than 81 kg. has been identified as an area that needs to be explored. The delivered dose of hemodialysis in large patients with end stage renal disease (ESRD), as measured by urea reduction ratio (URR) and urea clearance normalized for the volume of distribution of urea (Kt/V(urea)), is often less than adequate. See Carroll CE, et al., "Patient Factors Associated with Delivered KT/V in the U.S.", J Am Soc Nephrol 6:594, 1995. The 1995 ESRD Core Indicators project found statistically significant differences in urea reduction ratio (URR) in patients with a mean body weight of 81 kg. or greater. Current dialysis modalities do not take into account the growing number of patients fitting into this category. Use of large surface area dialyzers, high blood flow rates, high dialyzate flow rates and increased dialysis time are maneuvers that can increase the delivered dose of hemodialysis. Despite optimization of these parameters, patients weighing greater than 80 kg are still at risk of receiving less than adequate doses of hemodialysis, resulting in fatigue, tingling of extremities and changes in skin color of the patient.
It is known from the article by V. Albertini, et al., "Serial and Parallel Pairs of Dialyzers for High-Efficiency Treatment", in Kidney International 31(1), page 247, 1987, to arrange serially or in parallel two blood purifiers to enhance the purifying action. A means for precisely regulating and monitoring the filtrate and substituate flow is not mentioned in this publication. A recent report of two F-80 dialyzers used in series has demonstrated increased urea clearance when used in a system for hemodiaffiltration. See Bosch JP, et al., "Hemodialysis High Efficiency Treatments: Six Year Outcomes", J. Am. Soc. Nephrol. 6:484, 1995. However, such systems require machine alterations which would negate the warranty and some of the inherent safety features.
U.S. Pat. No. 5,660,772 also discloses a hemodiafiltration apparatus with two blood filters connected in series. However, this system requires a pump between the first and second blood filters.